In this chapter the site-specific data relevant to the UXO risk assessment are provided.
BATHYMETRY
To the Southwest of Bornholm, a shallow water area with AdIer Grund and Rønne Banke separates the Arkona and Bornholm Basins. The Water depths on Rønne Banke is about 20m, and on Adler Grund the shallowest area is about 10 m deep. The maximum water depth in the Bornholm Basin is 92m and the average depth in the Arkona Basin is 48m. In the Bornholm I West OWF site the water depth increases towards the northwest, from approximately 35m to 45m.
An overview of the bathymetry is shown in Figure 9.
Figure 9: Bathymetry overview. 100
SEABED SEDIMENTS
The distribution of seabed sediments reflects to some degree the bathymetry of the region. Fine-grained mud has accumulated in the central parts of the basins, whereas areas with till and bedrock in the shallow parts indicate non-deposition or erosion. In the Bornholm I West OWF site, sand is found in the shallowest parts of the site, with muddy sand dominating the area. Small areas mapped as mud, clay and silt are also found.
100 Source: Geological Survey of Denmark and Greenland (https://eng.geus.dk/, accessed October 8, 2021).
According to vibrocore data, clayey till is found at the seabed or close to the seabed in some parts of the site. Also, late glacial clay deposited in the Baltic Ice Lake is found in the northern part of the site.
Near landfall of the export cable corridor, sedimentary rock is present (outcropping Mesozoic deposits).
An overview of the seabed sediments is shown in Figure 10.
Figure 10: Seabed sidiments overview. 101
HYDRODYNAMICS IN THE AREA OF INVESTIGATION
The Baltic Sea is a semi-enclosed marginal sea consisting of a series of sills and basins. Shallow and narrow connections at the entrance between the Danish islands and Sweden limit the water
exchange with the North Sea. This leads, together with the distinct thermo- and haloclines to a highly stratified water column and the presence of gravity currents.
The hydrodynamics of the Baltic Sea are highly dependent on the salinity exchange with the North Sea. There are two distinct surface and bottom flow layers with significant variations in salinity and temperature. The surface layer is dominated by the low salinity freshwater inflows from the rivers and the bottom layer by saline water. The bottom layer transports the saline and warmer waters of the Belt Sea and Kattegat into the Arkona Basin.
101 Source: Geological Survey of Denmark and Greenland (https://eng.geus.dk/, accessed October 8, 2021).
Near bottom flow velocities range from 0 to approximately 60cm/s-1 in the main flow direction. 102 The wind speeds have a strong seasonal character with standard deviations that are roughly half of the mean values. The maximum wind speed range is 20-25m/s that mostly occurs during the autumn periods. The dominant wind direction is from the southwest. 103
The wave climate has a combination of the relatively modest long-term average wave height (0.76m) and the heights of most typical seas (0.25–0.5m) with the predominance of relatively short waves. A specific feature is the narrow range for typical wave periods (2.6s - 4s). Extreme wave heights of the order of HS ≈ 4m occur on average once a decade. 104
SEABED MORPHOLOGY
The seabed of the Bornholm I West OWF site is flat, without any distinct morphological features that may impact UXO migration and/or burial. The seabed is subject to sediment accumulation. The sedimentation rate in the Arkona basin was calculated for anthropogenically undisturbed mud cores for a period over 100 years. The accumulation rate was calculated to be 2.3 - 2.9mm/year. 105
UXO MIGRATION ASSESSMENT
In this paragraph the potential for horizontal UXO migration within the area of investigation is assessed. The result will be used as input for determining the ALARP certification requirements (e.g., survey corridor width, ALARP certificate validity).
3.5.1 Hydrodynamics
Past experiences have shown that man-made objects, such as UXO, can move around on the seafloor because of extreme weather events. Especially small calibre UXO in sandy energetic environments are known to migrate. 106 Research has demonstrated that even heavy objects (500 kg cylinders, similar to the characteristics of naval ground mines) may shift under strong enough wave conditions.
107
102 Lass, H. U. and Mohrholz, V. (2003): On dynamics and mixing of inflowing saltwater in the Arkona Sea, Journal of geophysical research, vol. 108, no. C2, 3042, doi:10.1029/2002JC001465, 2003.
103 Dargahi, B and Cvetkovic, V. (2014): Hydrodynamic and Transport Characterization of the Baltic Sea 2000-2009, TRITA-LWR.REPORT 2014:03, ISSN 1650-8610, ISBN: 978-91-7595-215-4.
104 Soomere, T. et al (2012): Wave climate in the Arkona Basin, the Baltic Sea, Ocean Science 8, 287–300, 2012
105 Bunke, D. et al (2019): Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea, Front. Mar. Sci., 12 November 2019
106 Traykowski, Peter (2015): MR-2319 Continuous Monitoring of Mobility, Burial, and Re-exposure of Underwater Munitions in Energetic Near-Shore Environments. Appendix D. in: Second Workshop on Burial and Mobility Modelling of Munitions in the Underwater Environment (2015), Final Report, SERDP
107 Papili, Sonia; Thomas Wever et al. (2014): Storm influence on the burial of objects in a shallow sandy shelf environment. Marine Geology, DOI: 10.1016/j.margeo.2014.01.004, Guyonic, Stéphane; Mathieu Mory et al. (2007): Full-Scale Mine Burial Experiments in Wave and Current Environments and Comparison with Models. IEEE Journal of Oceanic Engineering, DOI: 10.1109/JOE.2007.890951 and Bower, Grant R.; Michael D. Richardson et al. (2007): Measured and Predicted Burial of Cylinders During the Indian Rocks Beach Experiment. IEEE Journal of Oceanic Engineering, DOI: 10.1109/JOE.2007.890950
Offshore experiments showed that the largest movements of UXO surrogates occurred in the first two days after deployment on the seabed. In this stage scour related burial is still minimal, making the UXO particularly susceptible to mobility if sufficiently large waves occur. The high rate of initial migration was observed to be abruptly halted by burial lock-down.
Once a UXO is fully buried, subsequent movement is assessed to only be possible if bottom profile variation result in re-exposure to a sufficient degree that releases the UXO from burial lock-down and permits it to undergo additional scour and roll progressions. 108
In Germany, the requirements of initial movement of objects on the sea floor were investigated and a model was developed and validated allowing prediction of the incident fluid velocity that is necessary for an inertial motion of defined cylindrical and spherical objects. 109 Conservative assumptions were made on the critical near bottom current velocities needed to move certain UXO objects at various degrees of burial. The items considered were:
- British Depth Bomb Mark I.
- British 250 lb General Purpose bomb.
- German EMA mine (British designation GU).
- German EMC mine (British designation GY).
The critical near bottom current velocities assumed are displayed in Table 12 for various degrees of burial.
Item
Critical near bottom current velocity [m/s-1] 5% burial 15% burial 30% burial 50% burial
British Depth Bomb Mark I 1.2 1.5 1.9 2.2
British 250 lb GP bomb 1.6 2.0 2.4 2.7
German EMA mine 1.8 2.1 2.5 3.3
German EMC mine 2.2 2.7 2.9 3.9
Table 12: Critical near bottom current velocities for various stages of UXO burial. 110
The table shows that current velocities required for migration increase as the objects and the degree of burial increases.
The Bornholm I West OWF site is dominated by muddy sand. UXO deployed during World War I and II are likely to have become largely buried (> 50% of the UXO diameter) because of the UXO weight, the assumed bearing strength of the muddy seabed sediments and proceeding sediment accretion (up to approximately 30cm/100a). 111 This means that near bottom current velocities exceeding 2.2m/s-1 are required to uncover the UXO and potentially cause migration.
108 Wilson, Jeffry V., et al (2008): Predicting the Mobility and Burial of Underwater Munitions and Explosives of Concern Using the VORTEX Model, ESTCP Project MM-0417.
109 Menzel, Peter, et al (2017): Laboratory experiments and numerical simulations on the wave- and flow-induced migration of munition from WW1 and WW2 as a risk assessment for offshore construction.
110 Idem.
111 Bunke, D. et al (2019): Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea, Front. Mar. Sci., 12 November 2019.
The gravity current induced near bottom current velocities are in the order of decimetres per second.
Also, the water depths at the OWF site and the extreme wave heights (HS ≈ 4m) are such that wave induced currents will not result in above threshold current velocities (> 2.2m/s-1). Therefore, it is concluded that seabed currents are not sufficient to cause migration of UXO.
3.5.2 Morphodynamical behaviour
The area of investigation appears to be relatively stable (see paragraph 3.4). Mobile bedforms (e.g., sand waves) and migrating tidal channels are absent. The seabed in the Arkona basin is subjective to slow sediment accretion. The rate of sediment accretion is estimated to be up to approximately 30cm/100annum. 112 Therefore, UXO migration by morphodynamical processes can be excluded.
3.5.3 Human activity
Human activity will have a more significant impact on UXO migration than natural causes. Specifically fishing activities have the capacity to move items of UXO over considerable distances.
Large areas of the Baltic Sea are affected by bottom trawling. This also applies to the Arkona basin.
Beam and otter trawlers use strong outrigger booms to tow their fishing gear. These techniques are used to fish for shrimp and flatfish. Beam trawlers are known to also trap UXO accidentally in their nets. This is because the beam is intrusive to the seabed through contact of the gear components with the sediment. In the soft sediments the trawl marks remain visible for a long time and the furrows may reach a penetration depth of up to 30cm. 113 The UXO in the area of investigation are expected to only be partly buried, making them potentially susceptible to entrapment in fishing gear.
Local fisherman on Bornholm state that the area of investigation is not trawled often due to the presence of numerous boulders. This is confirmed by the results of the preliminary survey data showing lots of boulder fields. This will limit the potential for unintentional relocation of UXO.
It is not possible to quantify the UXO migration due to human interaction. Therefore, human interaction is not a factor in the ALARP sign off certification process. This migration factor is part of the baseline residual risk.
3.5.4 Conclusion
Based on the assessed information, UXO migration by natural causes can be excluded. The only factor possibly resulting in UXO migration is through human intervention. This factor is considered part of the baseline residual risk.
UXO BURIAL ASSESSMENT
In dynamic sediment conditions, UXO items are likely to become buried. The depth of burial is dependent on several variables that will be explored below.
3.6.1 Burial on impact
The first mechanism for UXO burial to consider is that due to initial impact. Burial on impact is applicable to air dropped UXO (e.g., air dropped bombs and ground mines).
112 Bunke, D. et al (2019): Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea, Front. Mar. Sci., 12 November 2019.
113 Krost, P., et al (1990): Otter trawl tracks in Kiel Bay (Western Baltic) mapped by side-scan sonar, Meeresforschung 32, 344–353.
In the marine environment, a bomb’s kinetic energy is rapidly attenuated by the water it passes through and its geometry is changed substantially. The depth of water, therefore, is a crucial factor in estimating the likely burial depth on impact.
Experiments on Mk84 bombs in the USA show that the trajectory of a bomb falling into water at an angle of entry of ~90° is rapidly altered by the new medium. The bomb rotates and orientates to near parallel to the seabed by a water depth of around 5-6m. Its burial in sandy soils due to impact will be minimal in water depths over 5m. 114 In muddy soils a bomb/ground mine may get buried because of the weight of the UXO, causing it to slowly sink down in the mud.
Figure 11: Comparison between modelled and observed Mk84 bomb trajectories. 115
Because of the water depths at the OWF site and most of the export cable corridor, burial on impact can be excluded. Potentially, in water depths under 5m, UXO burial on impact is possible. However, at the export cable landfall location, the seabed consists of sedimentary rock (see paragraph 3.2).
Based on the seabed conditions in the area with water depths under 5m significant UXO burial can be excluded.
3.6.2 Scour
Scour related burial is to be expected in non-cohesive sediments that are exposed to tidal or wave induced currents. Experiments and modelling have shown this depth to be approximately 0.6 times the diameter for large objects in sandy sediments. 116
114 Based on: Chu P.C. et al (May 2008): Semi Empirical Formulas of Drag/Lift Coefficients for High Speed Rigid Body Maneuvering in Water Column.
115 Chu, P.C. et al. (2010): Underwater Bomb Trajectory Prediction for Stand-off Assault (Mine/IED) Breaching Weapon Fuse Improvement (SOABWFI).
116 Based on: Douglas L. Inman et al., Scour and burial of bottom mines, A Mine Burial Primer, September 2002.
At the OWF site and in most of the export cable corridor, the currents are assessed to be too low to cause scour related burial of objects such as UXO. In the part of the export cable corridor that is subjective to wave induced currents the seabed consists of sedimentary rock. Here, scour related burial will not occur.
3.6.3 Bedform migration
In the area of investigation large migrating bed forms are absent. Therefore, UXO burial by bedform migration can be excluded.
3.6.4 Conclusion
Based on the assessed information the UXO burial depth is estimated to be a maximum of 1m below seabed.
In areas with sedimentary rock within 1m below seabed UXO burial is limited to the depth of the top of the sedimentary rock layer.